Mr. Vivian Lewes read a paper on "_Pentathionic Acid_." In March last the author, at the suggestion of Dr. Debus, undertook an investigation of pentathionic acid, the existence of which has been denied. The analyses of the liquid obtained by Wackenroder and others, by passing sulphureted hydrogen and sulphur dioxide through water, are based on the assumption that only one acid is present in the solution, and consequently do not establish the existence of pentathionic acid; as, for example, a mixture of one molecule of H_2S_4O_6 and one molecule of H_2S_6O_6 would give the same analytical results as H_2S_5O_6. Moreover, no salt of pentathionic acid has been prepared in a pure state. The author has succeeded in preparing barium pentathionate thus: A Wackenroder solution was about half neutralized with barium hydrate, filtered, and the clear solution evaporated _in vacuo_ over sulphuric acid. After eighteen days crystals, which proved to be barium pentathionate + 3 molecules of water, formed. These crystals were separated, and the liquid further evaporated, when a second crop was obtained intermediate in composition between the tetra and pentathionate. These were separated, and the mother-liquor on standing deposited some oblong rectangular crystals. These on analysis proved to consist of baric pentathionate with three molecules of water. This salt dissolves readily in cold water; the solution is decomposed by strong potassic hydrate, baric sulphite, hyposulphites, and sulphur being formed. By a similar method of procedure the author obtained potassium pentathionate, anhydrous, and with one or two molecules of water. The author promises some further results with some other salts of the higher thionates. The president said that the society had to thank the author for a very complete research on the subject of pentathionic acid. He, however, begged to differ from him as to his statements concerning the researches of Messrs. Takamatsu and Smith; in his opinion these authors had proved the existence of pentathionic acid. He hoped that the crystals (which were very fine) would be measured. Dr. Debus said that no one had previously been able to make the salts of pentathionic acid, and expressed his sense of the great merit due to the author for his perseverance and success. The paper opened up some highly interesting theoretical speculations as to the existence of hexathionic acid. If potassium tetrathionate was dissolved in water it could be re-crystallized, but potassium pentathionate under similar circumstances splits into sulphur and tetrathionate; but a mixture of tetrathionate and pentathionate can be re-crystallized. It seemed as if the sulphur when eliminated from the pentathionate combined with the tetrathionate.